3,216 research outputs found
Melting of hexagonal skyrmion states in chiral magnets
Skyrmions are spiral structures observed in thin films of certain magnetic materials (Uchida et al 2006 Science 311 359–61). Of the phases allowed by the crystalline symmetries of these materials (Yi et al 2009 Phys. Rev. B 80 054416), only the hexagonally packed phases (SCh) have been observed. Here the melting of the SCh phase is investigated using Monte Carlo simulations. In addition to the usual measure of skyrmion density, chiral charge, a morphological measure is considered. In doing so it is shown that the low-temperature reduction in chiral charge is associated with a change in skyrmion profiles rather than skyrmion destruction. At higher temperatures, the loss of six-fold symmetry is associated with the appearance of elongated skyrmions that disrupt the hexagonal packing
Emergence of skyrmion lattices and bimerons in chiral magnetic thin films with nonmagnetic impurities
Skyrmions are topologically protected field structures with particlelike characteristics that play important roles in several areas of science. Recently, skyrmions have been directly observed in chiral magnets. Here, we investigate the effects of pointlike nonmagnetic impurities on the distinct initial states (random or helical ones) and on the formation of the skyrmion crystal in a discrete lattice. Using Monte Carlo techniques, we have found that even a small percentage of spin vacancies present in the chiral magnetic thin film considerably affects the skyrmion order. The main effects of impurities are somewhat similar to thermal effects. The presence of these spin vacancies also induces the formation of bimerons in both the helical and skyrmion states. We also investigate how adjacent impurities forming a hole affect the skyrmion crystal
Kinetically Inhibited Order in a Diamond-Lattice Antiferromagnet
Frustrated magnetic systems exhibit highly degenerate ground states and
strong fluctuations, often leading to new physics. An intriguing example of
current interest is the antiferromagnet on a diamond lattice, realized
physically in A-site spinel materials. This is a prototypical system in three
dimensions where frustration arises from competing interactions rather than
purely geometric constraints, and theory suggests the possibility of unusual
magnetic order at low temperature. Here we present a comprehensive
single-crystal neutron scattering study of CoAl2O4, a highly frustrated A-site
spinel. We observe strong diffuse scattering that peaks at wavevectors
associated with Neel ordering. Below the temperature T*=6.5 K, there is a
dramatic change in the elastic scattering lineshape accompanied by the
emergence of well-defined spin-wave excitations. T* had previously been
associated with the onset of glassy behavior. Our new results suggest instead
that T* signifies a first-order phase transition, but with true long-range
order inhibited by the kinetic freezing of domain walls. This scenario might be
expected to occur widely in frustrated systems containing first-order phase
transitions and is a natural explanation for existing reports of anomalous
glassy behavior in other materials.Comment: 40 pages, 9 figures, Introduction and discussion altered and
expanded. Additional section and figure added to Supplementary Informatio
HAADF-STEM block-scanning strategy for local measurement of strain at the nanoscale
Lattice strain measurement of nanoscale semiconductor devices is crucial for
the semiconductor industry as strain substantially improves the electrical
performance of transistors. High resolution scanning transmission electron
microscopy (HR-STEM) imaging is an excellent tool that provides spatial
resolution at the atomic scale and strain information by applying Geometric
Phase Analysis or image fitting procedures. However, HR-STEM images regularly
suffer from scanning distortions and sample drift during image acquisition. In
this paper, we propose a new scanning strategy that drastically reduces
artefacts due to drift and scanning distortion, along with extending the field
of view. The method allows flexible tuning of the spatial resolution and
decouples the choice of field of view from the need for local atomic
resolution. It consists of the acquisition of a series of independent small
subimages containing an atomic resolution image of the local lattice. All
subimages are then analysed individually for strain by fitting a nonlinear
model to the lattice images. The obtained experimental strain maps are
quantitatively benchmarked against the Bessel diffraction technique. We
demonstrate that the proposed scanning strategy approaches the performance of
the diffraction technique while having the advantage that it does not require
specialized diffraction cameras
Perturbation Analysis of the Kuramoto Phase Diffusion Equation Subject to Quenched Frequency Disorder
The Kuramoto phase diffusion equation is a nonlinear partial differential
equation which describes the spatio-temporal evolution of a phase variable in
an oscillatory reaction diffusion system. Synchronization manifests itself in a
stationary phase gradient where all phases throughout a system evolve with the
same velocity, the synchronization frequency. The formation of concentric waves
can be explained by local impurities of higher frequency which can entrain
their surroundings. Concentric waves in synchronization also occur in
heterogeneous systems, where the local frequencies are distributed randomly. We
present a perturbation analysis of the synchronization frequency where the
perturbation is given by the heterogeneity of natural frequencies in the
system. The nonlinearity in form of dispersion, leads to an overall
acceleration of the oscillation for which the expected value can be calculated
from the second order perturbation terms. We apply the theory to simple
topologies, like a line or the sphere, and deduce the dependence of the
synchronization frequency on the size and the dimension of the oscillatory
medium. We show that our theory can be extended to include rotating waves in a
medium with periodic boundary conditions. By changing a system parameter the
synchronized state may become quasi degenerate. We demonstrate how perturbation
theory fails at such a critical point.Comment: 22 pages, 5 figure
Feedback control of inertial microfluidics using axial control forces
Inertial microfluidics is a promising tool for many lab-on-a-chip
applications. Particles in channel flows with Reynolds numbers above one
undergo cross-streamline migration to a discrete set of equilibrium positions
in square and rectangular channel cross sections. This effect has been used
extensively for particle sorting and the analysis of particle properties. Using
the lattice Boltzmann method, we determine equilibrium positions in square and
rectangular cross sections and classify their types of stability for different
Reynolds numbers, particle sizes, and channel aspect ratios. Our findings
thereby help to design microfluidic channels for particle sorting. Furthermore,
we demonstrate how an axial control force, which slows down the particles,
shifts the stable equilibrium position towards the channel center. Ultimately,
the particles then stay on the centerline for forces exceeding a threshold
value. This effect is sensitive to particle size and channel Reynolds number
and therefore suggests an efficient method for particle separation. In
combination with a hysteretic feedback scheme, we can even increase particle
throughput
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